1. Field of the Invention
The invention relates to an internal combustion engine with a laser ignition device, comprising a Q-switched, pumped solid-state laser with a pulsed pumped light source, a solid laser crystal embedded in a resonator, a Q-switch for increasing the power density, at least one output mirror and a focusing device, by means of which the laser beam may be focused in a combustion chamber.
The invention further relates to a Q-switched, pumped solid state laser, especially for a laser ignition device of an internal combustion engine, comprising a pulsed pumped light source formed by pump diodes, a solid laser crystal embedded in a resonator, a Q-switch for increasing the power density, at least one output mirror and a focusing device, with a cooling device being provided comprising at least one Peltier cooling element for cooling the resonator.
2. The Prior Art
Q-switched, pumped solid state lasers are especially suitable as laser ignition devices in internal combustion engines.
A laser ignition device for an internal combustion engine is known from U.S. Pat. No. 4,416,226 A, with the resonator of the laser plus photo-optical focusing device being screwed into a cylinder head bore in such a way that the ignition device opens directly into the combustion chamber. The laser ignition device applies the principle of a solid state laser with a pulsed pumped light source. This leads to the advantage that high pulse energies can be achieved with a relatively low input of power. A flash lamp is used as a pumped light source. In order to increase the power density, an actively switchable Q-switch is used. In so-called “Q-switching”, the energy is stored in the laser cavity during the pumping process of the active medium and released during a very short emission period. This results in an extremely high-energy laser pulse. Actively switchable Q-switches come with the disadvantage however that a considerably complex circuitry is required for the control and that they are less suitable for rapid sequences of pulses. The photo-optical device of the known laser ignition apparatus comprises three lenses. In combination with the active Q-switch and the pumped light source formed by the flash lamp, the most serious disadvantage is that the device cannot be housed entirely in a component to be screwed into a spark plug shaft. The specification does not provide any information about the cooling of the laser crystal and the light source as required in pumped solid state lasers.
U.S. Pat. No. 6,413,077 B1 describes a laser ignition device in which several lasers are used, namely an excitation laser and an ignition laser. The pulses of the excitation laser and the ignition laser are added up by means of a Q-switch and the power density required for ignition is made available. This known ignition device comes with the disadvantage of a very high constructional complexity and requires a lot of overall space in order to be used instead of a spark plug in an internal combustion engine.
Although the need for compact laser ignition devices has existed for a long time, there have not been any concrete constructional proposals for compact laser ignition devices for internal combustion engines. The publications U.S. Pat. No. 4,434,753 A and DE 37 36 442 A only show purely schematic illustrations of ignition devices in this respect. Components such as cooling devices which usually require a lot of overall space are missing, which is why these systems are not yet suitable for practical use in internal combustion engines.
The use of laser ignition instead of spark ignition offers a number of advantages. On the one hand, the place of ignition plasma which can be chosen relatively freely does not require any material structure which might impair the combustion process. Moreover, the high ignition pressures as occur in gas engines are beneficial to laser ignition because the required pulse energy decreases at higher pressures. Even leaner mixtures can be achieved with laser ignition, thus enabling the achievement of very low NOx emission values.
It is known from literature that a laser focused to a sufficiently small focus diameter leads with sufficient intensity to plasma formation and to a local increase of temperature and thus to an ignition of an explosive mixture. For practical gas mixtures, the avalanche effect of free electrons is used predominantly for explaining the plasma formation. The effect is then virtually independent of the employed wavelength.
U.S. Pat. No. 5,673,550 A describes the ignition of fuel droplets under plasma formation within the fuel-air mist by means of a laser pulsed via a coherent light source.
It is known to use pump diodes in pumped solid state lasers. In comparison with flash lamps, pump diodes come with the advantage of higher efficiency. In the case of solid state lasers pumped with pump diodes, the problem occurs that the pump diodes can only be operated within a very narrow temperature band. Excessive temperatures would drastically reduce the service life of the pump diodes.
U.S. Pat. No. 5,187,714 A describes a laser-diode-pumped solid state laser, with a Peltier device being provided for cooling. Diode-pumped solid state lasers cooled with Peltier elements are known from the publications JP 11-002849 A, JP 10-200177 A, JP 09-232665 A, JP 04-157778 A and JP 03-041787 A.
Cooling alone by Peltier elements is not sufficient for the use as an ignition device in internal combustion engines. Furthermore, the cooling is subject to further difficulties by the demand that the laser ignition device should be constructed in the highest compact manner and should be housed in the spar plug dome of a cylinder head of an internal combustion engine.
It is the object of the present invention to provide a temperature-stabilized solid state laser which is suitable for practical use in internal combustion engines as a laser ignition device, which only requires little space and which can be used in internal combustion engines with only little constructional effort.